Process for forming titanium

ABSTRACT

A process for forming titanium and titanium alloys comprising a two-step forming process of a partial form at sub-zero temperatures followed by a final form at room temperature or above.

United States Patent Collins [451 Apr. 4, 1972 PROCESS FOR FORMINGTITANIUM James D. Collins, Indianapolis, Ind.

Assignee: General Motors Corporation, Detroit,

Mich.

Filed: Apr. 2, 1970 App1.No.: 25,247

Inventor:

US. Cl. ..72/364, 72/700, 148/ 1 1.5,

148/125 Int. Cl. ..B21d 21/00, B21d 1/06 Field of Search ..72/364, 700;148/115, 125

Primary Examiner-Lowell A. Larson Attomey-Sidney Carter and Peter P.Kozak [57] ABSTRACT I A process for forming titanium and titanium alloyscomprising a two-step forming process of a partial form at sub-zerotemperatures followed by a final form at room temperature or above.

5 Claims, No Drawings PROCESS FOR FORMING TITANIUM This inventionrelates to metallurgical processing and particularly to a metallurigicalprocess for forming titanium. More particularly, this invention relatesto a two-step forming process to achieve increased formability oftitanium and titanium alloys at temperatures substantially below thosenormally required to form titanium.

In recent years with the advent of high speed aircraft and highoperating temperature gas turbine engines, titanium and titanium alloyshave assumed a predominant position in materials for use in structuralcomponents of aircraft because of their high strength-to-weight ratioand their resistance to the high temperatures normally occurring duringthe operation of high speed aircraft.

Titanium alloys are classed as a hard-to-form material because of theirhigh yield strength, which is on the order of 120 to 190 ksi. Because oftheir resistance to deformation, forming operations commonly take placeat greater loads than would be required for lower strength alloys and attemperatures above l,000 F. Several gas turbine engine components andstructural components of aircraft require severe bending of hard-to-formalloy sheets to keep the leading edges as sharp as possible to maintainmaximum gas flow efficiency. In addition, the tighter the bend that canbe made in a structural element, the more efficient, i.e. lighter, itwill be. For example, some engine designs require members to be bentaround a radius equalling the thickness of the sheet. This type offorming operation requires a 33 percent elongation of the outer fiber ofthe sheet and, therefore, when forming titanium alloys, temperatures of1,200" to l,500 F. are recommended. For forming operations employing aradius of twice the thickness of the sheet, which is the equivalent of a20 percent elongation of the outer fiber, forming temperatures of 1,200P are recommended. Room temperature forming operations are recommendedonly when the bend radius is greater than or equal to 4.5 times thethickness of the sheet which is equivalent to a 14 to 16 percentelongation or less of the outer fiber. The degree of bend radiicriticality can be reduced for a given forming operation if the formingis carried out in progressive stages with an anneal between stages.However, high temperature forming operations are undesirable becausethey result in excessive oxidation of the sheet and thinning of thesheet section at points of maximum deformation and because they requireheating of the forming dies to excessively high temperatures whichdrastically decreases the life of the dies. The interstage annealtechnique is also undesirable both for economic reasons and because ofthe progressive contamination of the material resulting from longexposure to annealing temperatures.

Accordingly, it is among the principal objects of my invention toprovide a method for facilitating the formability of titanium andtitanium alloys.

It is a further object of my invention to provide a process for formingtitanium and titanium alloys at temperatures substantially below thosenormally required to achieve deformation.

It is another object of my invention to provide a two-step formingprocess wherein a titanium or titanium alloy workpiece is partiallyformed at a low temperature followed by a final form at room temperatureor a moderately elevated temperature, thereby eliminating the need forinterstage thermal anneals and the need for the excessively high formingtemperatures normally required for forming titanium.

lt is yet another object of my invention to provide a two-step formingprocess wherein the titanium or titanium alloy workpiece is firstpartially formed at a sub-zero temperature and then finished formed at amoderately elevated temperature whereby increased formability at alesser load than is normally required for forming titanium is achieved.

ln accordance with the preferred embodiment of my invention, these andother objects are accomplished by first submerging a titanium andtitanium alloy workpiece in a bath of liquid nitrogen and partiallyforming the workpiece to about percent of the total deformation requiredand thereafter finish forming the workpiece at a temperature of 260 F.Both the loads and temperatures required for final deformation of thetitanium workpiece are substantially below those normally required intitanium forming operations and, therefore, my two-step forming processembodying my invention may be viewed as a work softening" process.

Other objects and advantages will be apparent from the followingdetailed description of my invention. As used hereinafter the termtitanium includes both titanium and titanium alloys containing at leastabout percent titanium.

A few general comments may first be made concerning my work softeningprocess. Work softening is of interest because it offers the possibilityof increasing the formability of titanium by a two-step operation ofplastic deformation at a low temperature followed by further deformationat room temperature or above without the requirement of an interstageanneal to remove accumulated work hardening or the excessively highworking temperatures normally required for forming titanium. Atheoretical requirement of the work softening mechanism is that thesecond deformation load at room temperature following the partialdeformation at a low temperature will not reach a normal load pointdescribed by a room temperature stress-strain curve. This constitutes afailure of the mechanical equation of state. In my investigations, thetwo-step work softening process has shown a considerable drop of theflow stress on further straining at the higher temperature. In addition,the low temperature work hardening has been shown to be removed morecompletely by the second deformation than could be accomplished by athermalanneal. This reduced load and dramatic improvement in formabilitysatisfies the work softening requirement. Although the theoreticalexplanation of the work softening mechanism is not known, I believe, butwithout limitations thereto, that the phenomenon can be described as acatastrophic release of glide dislocations which are piled up againstsessile dislocations at the low temperature to create an unstablecondition at the higher temperature.

In the practice of my invention, I prefer to use liquid nitrogen as amedium in which to cool the workpiece for the low temperature formingoperation because of its relatively low cost and ease of handling.However, other suitable liquified gases or cryogenic fluids known in thecurrent cryogenic art may be used. When liquid nitrogen is used, aholding tank may be suitably modified to contain the liquid and preventits evaporation. After cooling in the liquid nitrogen medium, theworkpiece is positioned in the dies of a forming apparatus to perform adesired preforming operation such as bending, stretching, crimping,rolling, pressing and the like at the temperature of the medium.

In the method of my invention, it is necessary to perform a certainamount of deformation in order for the work softening mechanism tobecome operative. Investigations of work softening in single crystalshas shown that a deformation of 5 to 6% is needed for the mechanism tobecome operative. However, in forming polycrystalline material, I findit preferable to have at least about a 10 percent deformation. Thedeformation may be continued above 10 percent with the amount ofallowable deformation being limited only by that at which cracking orcatastrophic failure of the workpiece is imminent. Although, it is not arequirement that the type of deformation, i.e., bending, stretching,etc., be the same as that at the second forming temperature since it isbelieved that the first forming operation is necessary only to produce adesirable dislocation arrangement, in a production operation it isdesirable that the two deformations conform. That is, if the workpieceis to be bent to a desired configuration, partial bending may take placeat the liquid nitrogen temperature with the final bending operationoccurring thereafter at the second or moderately elevated temperature.

After the workpiece has been partially formed to 10 percent or greaterdeformation it is heated to room temperature, which is an environmentaltemperature ranging from about 65 to F, or heated to a moderatelyelevated temperature for the finish forming operation. It has been founddesirable that the second forming operation take place as soon aspossible after the partial forming operation in order to realize themaximum formability achieved by the two-step operation. It is thoughtthat time lapses of as little as an hour between the forming operationsallows the material to undergo a certain degree of recovery and it hasbeen shown that an hours lapse substantially detracts from theformability of the material. In the preferred embodiment, the secondforming operation is performed ata temperature of about 260 F forreasons which will hereinafter be fully explained. However, it will beapparent that even in the preferred embodiment, finish forming isaccomplished at only a moderately elevated temperature and one which issubstantially below the 1,000 to 1,500 F range normally required forforming titanium.

Reference to the following specific examples will further serve toillustrate my invention and contribute to the understanding thereof andwill demonstrate the effectiveness of my invention in facilitating theformability of titanium.

EXAMPLE I Bend test specimens having deminsions of inch X 2 inches witha thickness of 0.062 inch were cut from Ti-6Al-4V and Ti-5Al-2.5Snannealed sheet material. The former alloy was cut perpendicular to therolling direction of the sheet, and the latter alloy was cut parallel tothe rolling direction of the sheet. The Ti-6Al-4V material has a tensilestrength of 135 ksi and a yield strength of 120 ksi; and the Ti-5Al-Snmaterial has a tensile strength of 125 ksi and a yield strength of 120ksi. Bend test data, especially minimum bend radius, is one of the mostwidely reported criterion of sheet formability. Minimum bend radius isusually based on the smallest radius of 90 bend that can be made withoutapparent cracking under 10X magnification. The minimum room temperaturebend radius for Ti-6Al-4V is 4.5 to 6:, where t the thickness of thesheet; and for Ti-5Al-2.5Sn is 3.5 to 4.5 t.

The specimens were placed in a standard bend test machine having endloading points 1 inch apart and having a 0.125 inch center loadingmandrel. The mandrel gives a 2: bend and was chosen to assure less thana 90 bend at room temperature in the material treated according to mytwo-step operation. A temperature of l,000 F is normally recommended fora 2: bend in untreated material. The strain rate was 0.05 inch perminute. Table 1 lists the results of this test.

TABLE 1. Bend Test Data For Treated and Untreated at a 2: bend. Theaverage maximum bend angle of the treated Ti-6Al-4V material was 52percent greater than the average maximum bend angle at room temperatureand for the treated Ti 5Al-2.5Sn material the average maximum bend anglewas 33 percent greater than the average room temperature maximum bendangle. As previously mentioned, it has been found desirable to performthe second fonning operation as soon as possible after low temperatureforming. Referring to Table i, it may be seen that the Ti-5Al-2.5Snmaterial showed less forrnability than the Ti-6Al-4V. It is believedthat the disparity arises from the longer rest between formingoperations given the Ti-5Al-2.5Sn material than the Ti-6Al-4V materialwhich allowed for the material to undergo a degree of recovery therebydissipating the portion of the unstable dislocation configurationproduced by the low temperature operation and thereby detracting fromthe formability of the material. Thus, by finish forming shortly afterthe partial forming operation, the recovery reaction is not allowed totake place and the maximum benefits of my two-step process may berealized.

EXAMPLE ll not show increased ductility was anisotropic. Further studiesindicated that in the work softening material there is a high tendencyfor the twin plane, i.e., the (10:3) plane, to be parallel to therolling direction; and for the anisotropic material there is a tendencyfor the basal plane, i.e., the (00:1) plane to be parallel to therolling direction. This suggests that the texturemay be a significantfactor in the ductility differences observed.

However, it has been found that, suprisingly, this anisotropic texturingeffect can be eliminated by performing the second forming operationfollowing the liquid nitrogen forming step at 260 F rather than at roomtemperature. Therefore, the preferred embodiment of my invention is tocarry out a partial forming operation at liquid nitrogen temperatures,and thereafter, finish form at 260 F. Tables 11 and Ill show the resultsof tests performed on textured T1-6Al-4V and Ti-5Al 2.5Sn wherein thesecond deformation was made at 260 F.

Speclmens All specimens were prepared and tested according to therocedure outlined in Exam le 1.

Material Trwmm MAI-4v TABLE II. Forming Data for Ti-6Al-4V u a g 55Maximum Bend Angle Bend Angle A! Test Conditions Parallel to Transverseto Liquid Nitrogen Temperature 70 t 35 R R 56 52 0 mg Ir. 0 mg ir.

48 31 Maximum Bend Angle At 74 52 Room Temp. Bend 42 57 Room Temperature77 55 Liquid Nitrogen Bend 44 70 7a ss' 60 First Bend in Liquid NitrogenFollowed Maximum Bend Angle At Room I I5 77 H by Room Temp. BendTemperature Alter 25 Bend 1 l5 70 First Bend in Liquid Nitrogen Followedin Liquid Nitrogen 70 -H- By Bend at 260 F. 108 109 70 Bend at 260 F.108 82 65 Catastrophic Fracture Failure 15 minute rest between formingoperations TABLE III. Forming Data fOl Ti-5Al-2.5SI1 -H- 1 hour restbetween forming operations 70 Maximum Bend Angle Test ConditionsParallel to Transverse to Rolling Dir. Rolling Dir.

Room Temp. Bend 50 63' Liquid Nitrogen Bend 44 44 First Bend in LiquidNitrogen Followed by Room Temp. Bend 88 80 First Bend in Liquid NitrogenFollowed by Bend at 260 F 106 108 Bend at 260 F 53 59 It may be seen,then, from Tables II and Ill that finish forming at 260 F following aninitial preform at liquid nitrogen temperature results in greatlyincreased ductility in the material over that obtained at roomtemperature with the ductility being independent of the bend direction.Again, it may be seen that the material treated according to thepreferred embodiment of my invention realized bend angles in excess ofthe 90 maximum expected. It will be recognized that 260 F acts as anactivation temperature in my process and that forming operations maytake place above 260 F. However, the temperature should be kept belowabout 600 F because of the adverse effect on the material andforming'dies caused by temperatures above 600 F. Preferably thetemperature is kept below about 280 F. Thus, it is apparent that thetwo-step forming operation embodying my invention greatly facilitatesthe formability of titanium and allows for drastic deformation of thetitanium in forming processes at temperatures substantially below thosenormally required.

Although the preceding discussion has referred generally to the practiceof my invention in connection with the aircraft industry, I do notintend to limit my invention thereto; and although my invention has beendescribed in terms of specific embodiments, it is to be understood thatother forms may be adopted within the scope of my invention.

I claim:

1. The process for forming titanium material comprising cooling saidmaterial to liquid nitrogen temperature, partially forming said materialat said temperature to at least about 10 percent of the totaldeformation required in forming said material, and thereafter finishforming said material at a temperature between about room temperatureand 280 F.

2. The process defined in claim 1 wherein said finish forming operationis performed at about 260 F.

3. The process for forming a sheet metal part made of titaniumcomprising, introducing said part in a liquid nitrogen medium to reducethe temperature of said part to the temperature of said medium,partially forming said part at said temperature to at least 10 percentof the total defonnation required in forming said part, and thereafterfinish forming said part at a temperature between about room temperatureand 280 F.

4. The process defined in claim 3 wherein said finish forming operationis perfonned at about 260 F.

5. The process for forming a sheet metal part made of titaniumcomprising, introducing said part in a liquid nitrogen medium to reducethe temperature of said part to the temperature of said medium,partially bending said part at said temperature to at least 10 percentof the total bending required in forming said part, and thereafterfinish bending said part at a temperature between about room temperatureand 280 F.

2. The process defined in claim 1 wherein said finish forming operation is performed at about 260* F.
 3. The process for forming a sheet metal part made of titanium comprising, introducing said part in a liquid nitrogen medium to reduce the temperature of said part to the temperature of said medium, partially forming said part at said temperature to at least 10 percent of the total deformation required in forming said part, and thereafter finish forming said part at a temperature between about room temperature and 280* F.
 4. The process defined in claim 3 wherein said finish forming operation is performed at about 260* F.
 5. The process for forming a sheet metal part made of titanium comprising, introducing said part in a liquid nitrogen medium to reduce the temperature of said part to the temperature of said medium, partially bending said part at said temperature to at least 10 percent of the total bending required in forming said part, and thereafter finish bending said part at a temperature between about room temperature and 280* F. 